1. Field of the Invention
The present invention relates generally to feedback-controlled switching and, more particularly, to pulse oximetry having feedback-controlled LED switching where noise in a sense current and in LED drive current are reduced by switching a reference voltage in the feedback loop of an op-amp circuit.
2. Related Work
A pulse oximeter is a type of blood gas monitor which non-invasively measures an amount of saturation of oxygen in the blood. The saturation of oxygenated blood may be determined from the differentiated absorption for two plethysmographic waveforms measured at separate wavelengths. The two waveforms are typically produced by driving a visible red light-emitting diode (LED) and an infra-red LED to produce two lights that pass through a patient""s tissue, and then detecting the light on the same or an opposite side of the tissue using one or more photodetectors. Although most conventional oximeters use the red and infra-red LEDs, other devices such as surface emitting laser devices having different wavelengths may also be used, and the number of LEDs can vary according to the specific measurement application. For example, it is known to set a number of laser diodes to be equal to or greater than the number of blood analytes that are to be measured by an instrument
The two LEDs emit light at different wavelengths. The photodetector output signal indicates the attenuation of the two different wavelength lights after the lights pass through the patient""s body. In order to obtain a degree of consistency and ease of use, the photodetector is generally placed in a clip or similar device attached to the patient""s finger or earlobe. Attenuation of the lights is substantially constant except for the flow of blood. Thus, the constant attenuation due to the light passing through the patient""s skin and other tissue can be determined and filtered from the photodetector signal, thereby obtaining a signal representing the desired blood oxygen characteristics. Signals containing a component related to a patient""s pulse are known as plethysmographic waves and are used in blood gas saturation measurements. So, for example, the red/infrared ratio for waveforms at different wavelengths may be analyzed to obtain oxygenization values.
It is known to activate the red and infra-red LEDs during different time periods, where the two LEDs are cycled on and off alternately, in order to enable the photodetector to receive one signal at a time. As a result of generating LED pulse trains in a time-division manner, a composite time-division signal is then received by the photodetector. Alternatively, switching of LEDs may be related to other parameters such as maintaining a particular duty cycle without regard to time-division multiplexing (TDM). Various methods, not limited to TDM or to periodic switching, for modulating the LEDs can also be employed.
In order to increase the accuracy and resolution of the oximeter, it is desirable to reduce noise in the circuitry used to produce one or more drive currents for causing the LEDs to illuminate. Conventional LED drive circuits have been designed to minimize photic noise generated by the LEDs, in order to maximize a signal-to-noise ratio for the arterial attenuation signal(s) used in processing oximetry data. However, as is further discussed below, conventional LED driving circuits do not consider that a switching of a reference voltage may be a source of noise.
A typical apparatus employing a time-division diode driving scheme includes LED current drivers having a serial configuration where the outputs of two voltage-to-current converters are switched so that only one of two LEDs, connected in a back-to-back configuration, is on at any given time. The LED drive circuitry activates the red LED for a quarter cycle and activates the infra-red LED for a quarter cycle, with a quarter cycle xe2x80x9cdarkxe2x80x9d period separating each successive activation period. Since the two LEDs are on only periodically, less noise is generated from the LEDs and corresponding LED drive circuitry. This conventional LED drive circuitry uses ganged-type switch banks to alternately switch on/off both the input and the output of each voltage-to-current converter, thereby reducing noise from both the switching and voltage-to-current conversion circuitry. Both the reference voltage input and the resultant drive current output of a conventional dual LED drive circuit are simultaneously switched off because merely setting the reference set point to zero does not account for offset voltages in the op-amp that keep the op-amp in a xe2x80x9cturned-onxe2x80x9d state and that keeps the LED drive current turned on. The LEDs are thus driven to provide light transmission with digital modulation at a fixed low frequency f, where each period 1/f contains the aforementioned four quarter-cycle periods.
As shown in FIG. 3, a conventional LED drive circuit includes a reference voltage source 324 that generates an analog output, which is fed to a digital-to-analog (D/A) converter 325. The output of the D/A 325 is then output to a switch bank 326. When the switch bank 326 is in an ON condition, the D/A output signal is switchably connected to one of two voltage-to-current (V/I) converters 328, 329. When the switch bank 326 is in an OFF condition, the D/A output signal is switchably connected to the other of the two V/I converters 328, 329. The output of the currently activated V/I 328, 329 is connected to a pair of back-to-back LEDs 301, 302 via a second switch bank 327, which disconnects the output of the currently deactivated V/I converter 328, 329 from the LEDs 301, 302.
A conventional LED drive circuit, such as that described above, switches the reference voltage to each V/I converter. This conventional switching of the reference voltage creates a noise that is then amplified by the respective V/I converter. Although the conventional switching described above disables both the input and output of serially-oriented LED drivers, resulting in less overall average LED drive circuitry noise, it does not consider the noise created by the switching itself.
Oximetry noise is known to those of ordinary skill in the oximetry art to include any signal portions relating to ambient light, motion artifacts, absorption variance other than the plethysmographic effects of interest, electromagnetic radiation, electrical interference, magnetic fields, electronic interference such as harmonics or RF, and others.
Conventional voltage reference sources are chosen for use as a low noise DC voltage reference for a digital to analog conversion circuit 325. In that regard, the conventional voltage reference of FIG. 3 has a lowpass output filter (not shown) with a low corner frequency of 1 Hz. The digital to analog converter 325 also has a lowpass filter at its output with a similar low corner frequency of 1 Hz. The digital to analog converter 325 provides signals for each of the emitters 301, 302.
In the conventional FIG. 3 circuit, voltage to current converters 328, 329 can each have a feedback loop (not shown) that is configured to have a low pass filter to reduce noise. The low pass filtering function of the voltage to current converter 328, 329 has a corner frequency of just above 625 Hz, which is the switching speed for the emitters 301, 302.
Other filters (not shown) are typically used to reduce the effects of ambient electromagnetic noise in electronic monitoring instruments, especially when the noise source frequency (or a harmonic of the noise source frequency) is approximately the same as the fundamental frequency or harmonics at which the instrument is operating. In addition, a static filtering using a bandpass filter has been conventionally used to remove a portion of the photodetector""s output noise signal that is outside an identified bandwidth of interest, leaving random and/or erratic noise that is within the filter""s passband. A processor has conventionally been used to separate-out primary signal portions in order to isolate and identify the remaining noise signals, which are then removed using, for example, an adaptive noise canceller. Such a scheme is known as correlation canceling.
What is needed is a lower noise diode driving circuit, where a lower frequency low-pass filtering may be employed. An improved method in an oximeter for switching a diode current source reference voltage is needed, in order to eliminate noise caused by the switching itself.
The present inventor has recognized that noise due to an LED driving circuit can create cognizable artifact, particularly when oversampling type processing is used in oximetry systems. Switching noise in an LED current drive was conventionally not considered by designers of oximetry systems because typical processing systems were unable to xe2x80x9cseexe2x80x9d this switching noise being produced by a diode current driving circuit. The present invention uniquely maintains an ultra-low noise by eliminating a switching of a reference voltage being provided to an LED driving circuit.
In general, none of the prior art has considered the significance of noise in an oximetry system due to a diode current driving circuit. In particular, conventional assumptions regarding oximetry noise did not consider switching noise of an LED drive circuit, either because processing was unable to discern or distinguish noises as being due to the diode driving circuit, or simply because it was assumed that such noise could simply be filtered-out as estimated noise components when processing a detected signal. In addition, conventional oximetry systems have not attempted to provide a xe2x80x9ccleanxe2x80x9d LED drive circuit with expanded dynamic range. With state-of-the-art processing hardware and software, e.g., oversampling, a higher processing capability and greater resolution allows xe2x80x9cseeingxe2x80x9d smaller noises that were previously unnoticed. Along with improving resolution, it is of paramount importance to reduce noise sources in the LED drive circuitry of an oximeter rather than separating-out resultant signals during processing of signals from the oximeter""s photodetector. By using as clean a diode driving circuit as possible, less noise is present in downstream signals, and dynamic performance is improved, especially at low frequencies.
In order to improve over conventional LED drive systems, the present invention includes an apparatus and method for switching of one or more LEDs. The apparatus contains a switch, which turns the LEDs on and off, in the feedback loop of an op-amp driver circuit. This arrangement allows the current set point reference voltage to be directly connected to the op-amp, which reduces the noise of the LED drive circuit. A variation of the apparatus includes a variable resistance used as the current sensing resistor of the LED drive circuit.
A method according to the present invention includes placing a switch in the feedback loop of an op-amp LED driver circuit, and utilizing the switch for on/off switching a reference voltage to the op-amp in order to lower a noise current of the LED driver circuit. A variation of the method further includes changing the resistance of a current-sensing resistor to vary LED drive currents.
The present invention can be applied to any number of diodes being driven by a feedback type amplifier, so that a reference voltage is not switched but is directly applied to the amplifier, which has a switch in its feedback loop that controls the on/off switching of the diodes.
This summary does not limit the invention, which is instead defined by the appended claims.